Enzymes are extremely sensitive to acids, bases, or salts, their activity being often enormously enhanced or, in other cases, entirely inhibited, by the presence in the reacting medium of very small amounts of free acids, or bases, or even of certain neutral salts. For example, pepsin, the enzyme of the stomach will act only in the presence of a slightly acid medium and is wholly inactive in a mixture which contains even the slightest amount of free alkaline material; while trypsin, the similar enzyme of the intestine, acts only under alkaline conditions. Practically all enzymes are rendered inactive, but not destroyed, by the presence of either acid or alkali in excess of N/10 strength. Many will act only in the presence of small quantities of certain specific neutral salts; while, on the other hand, other salts are powerful inhibitors of enzyme action. Enzymes often differ from the protoplasm which secretes them in their response to antiseptics, such as toluene, xylene, etc., which inhibit the activity or growth of the cell, but have no effect upon the activity of the enzymes which it contains. THE CHEMICAL NATURE OF ENZYMES Nothing is known with certainty concerning the chemical nature of enzymes. Being colloidal in nature, they adsorb carbohydrates, proteins, fats, etc., so that active enzyme preparations often respond to the characteristic tests for these groups of substances; and many investigators have reported what has, at first, seemed to be conclusive evidence that some particular enzyme which they have studied is either a carbohydrate, a protein, or some other type of organic compound. Later investigations have always shown, however, that if the preparation in question be submitted to the digestive action of the enzymes which hydrolyze the particular type of substances to which it is supposed to belong, the material will lose its characteristic protein, or carbohydrate, etc., properties, without losing its specific activity, thus clearly indicating that the substance which responds to the characteristic tests for some well-known type of organic compounds is present as an impurity and is not the enzyme itself. The present state of knowledge concerning the nature of enzymes seems to indicate that, like the inorganic catalysts, they may vary widely in chemical composition; and that their tremendous catalytic effects are due, in part at least, to their colloidal nature. This will be better understood and appreciated after the phenomena associated with the colloidal condition have been considered (see the following chapter). NOMENCLATURE AND CLASSIFICATION Since nothing is known of the chemical composition of enzymes, they can only be studied by considering the effects which they produce. This is reflected in the systems which have been adopted for their nomenclature and classification.